Back light brightness of all in one (AIO) computers are adjusted by the central processing unit (CPU) in a host computer or by the display chip in a monitor. In the Windows 8 operating system, the back light brightness must be adjustable by the CPU so as to pass a windows hardware quality laboratory (WHQL) test of Microsoft company. When the AIO computer is only used as a monitor and communicates with peripheral devices, the back light brightness is adjusted by the display chip and cannot pass the WHQL test.